As the technical development of and the demand on mobile devices increase, the demand on secondary batteries as an energy source has rapidly increased, and use of the secondary batteries as a power source for electric vehicles (EVs) and hybrid electric vehicles (HEVs) has been recently realized. Among such secondary batteries, the demand for lithium secondary batteries with high energy density, high discharge voltage and output stability is high.
An electrode assembly constituting a secondary battery is largely classified into a jelly-roll type (wound type) and a stacked type (laminated type) according to the structure of the electrode assembly consisting of a positive electrode, a separator and a negative electrode. A jelly-roll type electrode assembly is manufactured by coating a metal foil used as a current collector with an electrode active material, drying and pressing, cutting the coated metal foil into a band shape having a desired width and length, and separating a negative electrode and a positive electrode using a separator, followed by spirally winding. Although the jelly-roll type electrode assembly is suitable for a cylinder type battery, the jelly-roll type electrode assembly has disadvantages such as peeling of an electrode active material and low space utilization when applied to a prismatic type battery or a pouch type battery. On the other hand, the stacked type electrode assembly has a structure in which a plurality of positive electrode and negative electrode units are sequentially stacked, and thus, the stacked type electrode assembly has an advantage of being easy to obtain a prismatic shape, but has disadvantages in that a manufacturing process is complicated and a short circuit is caused by an electrode being pushed when an impact is applied.
In order to solve these problems, as an electrode assembly having an advanced structure that is a mixed type of the jelly-roll type and the stacked type, an electrode assembly having a structure in which unit cells of a full-cell of a positive electrode/separator/negative electrode structure or a bi-cell of a positive electrode (negative electrode)/separator/negative electrode (positive electrode)/separator/positive electrode (negative electrode) structure of a certain unit size are folded by using a long continuous film of a separator has been developed and disclosed in Korean Patent Application Publication Nos. 2001-82058, 2001-82059, 2001-82060, and the like. In this application, an electrode assembly having such a structure is referred to as a stacked/folded type electrode assembly.
The stacked/folded type electrode assembly is manufactured through a winding process using a mandrel. Conventionally, since a first unit cell, which is located at a position in which winding is started, among unit cells arranged on a separator film is arranged in a state of being separated from a winding end portion of the separator film, a surplus winding end portion of the separator film is folded out toward a gripper of a mandrel in a winding process, and in this case, the surplus winding end portion may be not neatly folded during winding and a defect such as being folded in a straight line may occur.
In order to solve the above problem, a separator inner folding (SIF) process, in which a surplus winding end portion of a separator film near a position in which winding is started is folded inward toward a first unit cell located at a winding start point by using a blower method using wind, is applied.
FIG. 1 is a schematic view of a conventional folding device 10 in which the above described method is applied, and FIG. 2 is a schematic view for describing a folding phenomenon of a separator that occurs after the above described method is applied.
Referring to FIGS. 1 and 2, the folding device 10 includes a unit cell arrangement unit 12 configured to arrange unit cells 2, 3, 4, 5 and 6 at a predetermined interval on a separator film 1, a pair of rollers 15 configured to press the separator film 1 and the unit cells 2, 3, 4, 5, and 6, a blower 13 configured to inwardly fold a surplus winding end portion 11 of the separator film in a direction covering an upper surface of a first unit cell 2 by blowing wind to the surplus winding end portion 11 of the separator film at a lower portion of the separator film 1 which is opposite to a direction in which the unit cells 2, 3, 4, 5, and 6 are arranged, and a mandrel 14 configured to hold and rotate the separator film 1 together with the unit cells 2, 3, 4, 5, and 6 and so that the unit cells 2, 3, 4, 5, and 6 are sequentially stacked with the separator film 1 interposed therebetween.
However, even in such a structure, as shown in FIG. 2, a portion of the surplus winding end portion 11 of the separator film 1 may not be properly folded inwardly due to the specificity of a method using wind, and a separator folding (11-1) phenomenon still occurs, resulting in occurring a short circuit due to contact between the adjacent unit cells.
Therefore, there is a great need for a technique capable of completely solving the problem of the separator folding, reducing a defect rate, and improving the safety of a battery.